Research profile

Dr. Christian Papp studied Chemistry at the Friedrich-Alexander-Universität Erlangen-Nürnberg. He finished his PhD in the group of Prof. Hans-Peter Steinrück with summa cum laude in 2007; the title of the thesis was: “Model systems in heterogeneous catalysis”. Thereafter, he moved with a prestigious Feodor Lynen stipend of the Alexander von Humboldt foundation to work at Lawrence Berkeley National Laboratory and University of California, Davis with Prof. Charles S. Fadley. During his postdoctoral visit he conducted pioneering experiments on soft x-ray standing wave experiments and hard x-ray photoelectron spectroscopy of strongly correlated materials and spintronic systems. In 2009, he returned to Germany to become group leader of the “Surface and in situ spectroscopy group” at the Lehrstuhl für Physikalische Chemie II in Erlangen. His work focuses on in situ experiments primarily conducted with synchrotron radiation at BESSY II in Berlin.

His research is focused on the in situ analysis of surfaces and interfaces facilitating synchrotron radiation for

1)      the fundamental understanding of surface processes,

2)      materials characterization and synthesis,

3)      the analysis of heterogeneous catalysis,

4)      the development of new in situ techniques.

In the following his main achievements and projects of the past years are summarized.

Fundamental insights in surface reactions
These studies, performed with in situ high-resolution X-ray photoelectron spectroscopy at third generation synchrotron sources, resulted in an unprecedented level of insight to surface reactions. Various basic concepts of analyzing surfaces reaction were used and a deep understanding of the processes ongoing on the surface was achieved. The reactions studied range from the oxidation of sulfur and CO to C-H and C-C activation as well as C-C coupling on surfaces. The analysis allowed for determining various reaction intermediates, the reaction kinetics, kinetic isotope effects, activation energies and the rate determining step in the surface reactions. Also various spectroscopic details were evaluated with XPS such as the vibrational properties of adsorbates and reaction intermediates, adsorption sites, also “within” a molecule, were analyzed. For the analysis of surface reactions, new time resolved measurements methods were established. These studies are setting a standard for a thorough and complete molecular understanding of the surface reactions, with all reaction intermediates identified and the kinetics of the reaction determined, see e.g.

C. Papp, H.-P. Steinrück Surf. Sci. Rep. 68 (2013) 446.
R. Streber, C. Papp et al. Angew. Chem. Int. Ed. 48 (2009) 9925.

Liquid organic hydrogen carriers

Besides the fundamental understanding of the reaction of small molecules, the reactivity of larger molecules is investigated in the framework of the Cluster of Excellence ‘Engineering of Advanced Materials’ and a cooperation with BMW. Studying these particular systems is motivated by one of the grand present challenges of mankind, namely the storage of energy. Among several approaches, one potential solution is “chemical storage” of hydrogen using Liquid Organic Hydrogen Carrier (LOHC) materials. These substances are high boiling organic molecules, which can be reversibly hydrogenated and dehydrogenated using heterogeneous catalysts.
Despite the high relevance of such systems, the molecular level understanding of the catalytic dehydrogenation and hydrogenation of LOHCs is still at its infancy. This is partly due to the size of the molecules (forty and more atoms) which makes them a major challenge for surface science methods. Nevertheless, the Papp group demonstrated that by an in situ XPS study of such molecules on model catalysts, one can obtain detailed insights into the mechanisms of dehydrogenation and also of relevant side-reactions, at the molecular level. See e.g.

C. Gleichweit C. Papp et al. ChemSusChem 6 (2013) 974.
C. Papp et al. Chem. Rec. 14 (2014) 879.

P. Preuster, C. Papp et al. Acc. Chem. Res. 50 (2017) 74.

Growth and Chemical Modification of Graphene

Graphene, i.e., a single layer of graphite, is a promising candidate for future carbon-based electronics. Its high potential for applications stems from the specific electronic structure of freestanding graphene, which shows a linear dispersion at the Dirac point, indicating the existence of relativistic quasiparticles. The growth of graphene on metal surfaces has received significant attention as a potential low energy route for its production: the required growth temperatures are significantly lower than on SiC, but nevertheless high-quality, large area graphene sheets can be formed. In their investigations, Papp and coworkers synthesized graphene on transition metal surfaces and thereafter characterized the system with angle resolved photoemission and x-ray photoemission. Besides pristine graphene, also the synthesis of doped graphene layers, with dopants such as nitrogen and boron was demonstrated, using various approaches. For all cases, the band structure and thus the changes that occurred due to doping were studied. In the course of these studies, coexisting structures of graphene were observed on a nickel surface, and different doping, i.e. n-or p- type doping, were found, depending on the dopant geometry. As a further graphene-related topic, monodisperse metal nanoclusters grown on a graphene Moire were used as a model system for catalysis. Here the aim is to understand carbon-supported heterogeneous catalysis, with the possibility to tune the cluster chemistry by tuning the graphene electronic structure. See e.g. 

J. Englert, C. Papp et al. Nat. Chem. 3 (2011) 279.
R. Koch, C. Papp et al. Phys. Rev. B 86 (2012) 75401.
K. Gotterbarm, C. Papp et al. J. Phys. Chem. C 118 (2014) 15934.

C. Papp Cat. Lett. 147 (2017) 2683
F. Späth, C. Papp et al. 2D Materials 4 (2017) 35026

Near ambient pressure X-ray photoemission

A further in situ technique used in the group of C. Papp is near ambient pressure X-ray photoelectron spectroscopy (APXPS), a modern tool to study the surface of liquids and solid surfaces under ambient conditions. This technique allows insights in various areas, particularly in atmospheric, environment and catalysis sciences. NAPXPS adds important new information in the field of surfaces in the presence of gases and vapors, closing the gap between high pressure and ultra high vacuum conditions. The systems studied range from model catalysis systems such as the CO oxidation on platinum, to of novel heterogeneous catalysts as Pt nanoparticles on and in titania nanotubes, ethanol steam reforming on Co ceria systems, bimetallic catalysts such as Pt/Ga and to liquid systems as in the case of the CO2 capture reaction of functionalized ionic liquids. See e.g.

I. Niedermayer, C. Papp et al. J. Am. Chem. Soc. 136 (2014) 436.
S. Krick Calderon, C. Papp et al. Angew. Chem. Int. Ed. 56 (2017) 2594.
N. Taccardi, C. Papp et al. Nat. Chemistry 9 (2017) 862.

L. Ovari, C. Papp et al. J. Cat. 307 (2013) 132.

Hard X-ray photoemission and angle resolved hard X-ray photoemission

The use of hard X-rays in photoemission opens the new opportunities in the field of materials characterization. The inherent surface sensitivity of soft X-rays paved the way to investigate surface properties of novel materials; nevertheless, information on the bulk properties such as the electronic structure cannot be obtained easily. C Papp was involved in the pioneering experiments that showed that hard x-rays with a significantly higher escape depth of the photoelectrons allow to directly study buried interfaces, omnipresent in electronic devices, as e. g, gating materials, or the bulk electronic structure, with this new approach. See e.g.

A. Gray, C. Papp et al. Nat. Mat. 10 (2011) 759.
C. Papp et al. J. Appl. Phys. 112 (2012) 114501.

Soft X-ray standing waves

A second approach to study buried interfaces are soft X-ray standing wave experiments. This method, based on the hard X-ray standing wave technique, now allows for investigating systems that have dimensions, relevant to industrial applications, i.e. nanometers. Typical systems studied were strongly correlated materials, layered systems for spintronic applications, etc. See e.g.

S. H. Yang, C. Papp et al. Phys. Rev. B 84 (2011) 184410.
A. Gray, C. Papp et al. Phys. Rev. B 82 (2010) 205116.